In general, a pressure mount type semiconductor device such as a silicon diode or a thyristor comprises a base having a cylindrical wall provided on the top of a base body to contain a spring which exerts pressure on a semiconductor pellet. A pellet pressing member which may be in the form of a ring is engaged by a protrusion formed by diametrically and inwardly extending the cylindrical wall. The base is provided on the upper surface with an annular projection of weldable metal material which is used to projection weld a shell to cover a semiconductor pellet on the base. A conventional base of such a pressure mount type semiconductor device comprises a first base member formed by deforming a metal blank of electrically and thermally conductive metal material such as copper by extrusion and including a base body and a stud vertically extending from the lower surface of the base body, and a second or annular base member of weldable metal material such as steel having an annular projection formed on the upper surface of the annular base member by a cutter and also having a cylindrical wall extending vertically and upwardly from the inner edge of the annular base member, the second base member being silver-soldered to the first base member. As noted from the above description, the manufacture of the conventional base requires three troublesome steps, i.e. the extrusion of the first base member, cutting of the second base member and silver-soldering the first and second base members, and therefore, the effectiveness of the operation is unsatisfactory with the result that the base is expensive to manufacture.